1. Field of the Invention
The present invention relates to a down converter for use in satellite communication system and, more particularly, to a down converter for microwave digital satellite communication.
2 Description of the Prior Art
The Local signal used for frequency conversion by the down converter in a microwave digital satellite communication system requires a high quality in terms of frequency probability, frequency stability, phase noise characteristic or the like.
One prior art down converter DC' is described below with reference to the FIG. 2.
A microwave signal MS of a frequency such as 12 GHz is input to a microwave low noise amplifier 5 through a microwave input terminal 1. The low noise amplifier 5 amplifies the inputted signal MS and produces an amplified micro wave signal MSa. The amplified microwave signal MSa is input to a mixing circuit 6.
Through an input terminal 3, a standard signal SS' having a frequency in the order of MHz is inputted to a phase locked oscillator (PLO) 9 which produces a phase synchronized signal SSs' having a frequency such as 2.15 GHz. The phase synchronized signal SSs' is input to a frequency multiplier (FM) 8 which produces a local signal SSl' of a microwave band at a frequency range of ten GHz by multiplying the signal SSs' by, e.g. five (2.15 GHz.times.5=10.75 GHz). The local signal SSl' is input to the mixing circuit 6.
In the mixing circuit 6, a frequency difference between the amplified microwave signal MSa and the local signal SSl' is obtained to produce an intermediate frequency signal IF' having a frequency range of, for example, 950 MHz. The intermediate frequency signal IF' is input to an intermediate frequency amplifier 7 and is thereby amplified. The amplified intermediated frequency IFa' is produced from an output terminal 2. Through a DC source terminal 4, an electric power DC is supplied to a power source (PS) circuit 10 for driving the down converter DC'.
According to the prior art down converter DC', terminals 2, 3 and 4 are provided for connection with a next stage circuit component, such as a receiver. Thus, if a cable is used, a 3-line cable is necessary to connect terminals 2, 3, and 4 to the next stage component.
As a result, the conventional down converter DC' as described above requires a number of lines in a cable, or a number of separate cables are necessary for connection with the next stage component, resulting in difficulties of installation and maintenance, degradation of reliability, and the increase of the cost of the down converter.